We are studying temperature-sensitive mutants of polyoma virus to identify the viral gene functions affecting cell growth regulation. We will attempt to select mutants that may be blocked in a function required for cell transformation, but not for lytic growth of the virus. We will map a number of polyoma mutants by the use of restriction enzyme analysis. We will characterize a number of mutants wich may contain altered virion proteins to identify the viral genes coding for proteins which are found in the virion.